The present invention generally relates to optical type discs which are recorded with information signals on each sloping surface of a spiral V-shaped groove or concentric V-shaped grooves formed on a recording surface of the disc and optical type reproducing apparatuses for reproducing the information signals from such discs, and more particularly to an optical type disc which is not recorded with signals exclusively for tracking control and an optical type reproducing apparatus for reproducing the information signals from such a disc in a state where a satisfactory tracking control is carried out.
In a system which relatively scans over a spiral or concentric tracks formed on a recording surface of an optical type disc (hereinafter simply referred to as a disc) by use of a laser beam, for example, information signals are reproduced by detecting the laser beam which is reflected from the recording surface of the disc. Generally, the recording surface of the disc is flat, and a predetermined gap is formed between two mutually adjacent tracks so that a spot of the laser beam on the recording surface does not extend over a track which is adjacent to the track which is being scanned. The recording density of the disc is hence determined by the diameter of the spot of the laser beam on the recording surface. The laser beam is stopped to approximately the diffraction limit, and for this reason, it was thought that the recording density of the existing disc is the limit for this kind of disc.
Recently, various proposals have been made to increase the recording density of the optical type disc. Among such proposals, a disc having a recording density which is approximately twice the recording density of the existing disc was previously proposed in a Japanese Laid-Open Patent Application No. 57-147133. According to this previously proposed disc, a spiral or concentric V-shaped grooves are formed contiguously on the recording surface of the disc, and the information signals are recorded on each sloping surface of the V-shaped grooves. In other words, each V-shaped groove has two sloping surfaces, and each sloping surface constitutes a track. According to this previously proposed disc, the crosstalk from the adjacent tracks is relatively small, and for this reason, the interval between two mutually adjacent tracks can be set to a small value so as to effectively increase the recording density of the disc.
However, the previously proposed disc had problems in controlling the tracking of the laser beam over the correct track. In a first example, signals for tracking control (hereinafter referred to as tracking control signals) are recorded along the peak part and the valley part of each V-shaped groove, and the tracking is controlled responsive to reproduced tracking control signals. However, in this first example, an additional process is required to record the tracking control signals on the disc and the manufacturing cost of the disc becomes high. In addition, the recording density of the disc with respect to the information signals becomes reduced due to the provision of the tracking control signals, and the frequency band of the information signals becomes limited because of the need to reserve a frequency band for the tracking control signals. Further, the construction of the reproducing apparatus becomes complex because of the need to provide circuits for processing the reproduced tracking control signals. In a second example, the V-shaped groove is intentionally made to wobble with a certain frequency, where the quantity of each wobble is considerably small compared to the interval between two mutually adjacent tracks. According to this second example, the tracking control is carried out responsive to a wobbling component in the laser beam which is reflected from the recording surface. However, it is difficult to form such a wobbling V-shaped groove on the recording surface of the disc, and moreover, the construction of the reproducing apparatus becomes complex because of the need to provide circuits for processing the wobbling component.
On the other hand, the information signals are recorded on the optical type disc as variations in the reflectivity of a tellurium suboxide (TeOx) thin film, for example, which thin film is formed on each sloping surface of the V-shaped groove. Accordingly, each sloping surface of the V-shaped groove remains flat even after the information signals are recorded, and thus, it is impossible to duplicate and mass product the disc from an original disc by use of a stamper.
It is possible to duplicate and mass produce the disc from the original disc by use of the stamper, if the information signals were to be recorded on each sloping surface of the V-shaped groove as rows of pits. However, when the depth of the pits are approximately constant as in the case of the conventional disc of the type having the flat recording surface (that is, having no V-shaped groove), the problem of crosstalk occurs at the time of the reproduction as will be described later on in the present specification.